In the conventional art, there is known a method of direct measurement by withdrawal of the blood from the patient, particularly the blood from an artery, for detecting CO.sub.2 concentration in the blood. This method was, however, not suitable for continuous measurement and caused pains to the patient.
The method of transcutaneous measurement according to the present invention is different from the direct method described above. Because the present invention measures the carbon dioxide which is diffused from blood and through capillary blood vessels and skin, it does not cause any pain to the patient and is capable of continuously measuring the CO.sub.2 partial pressure of the patient.
Reference is now made to the accompanying drawings wherein:
FIG. 1 is a cross sectional view showing a basic structure of a transcutaneous carbon dioxide measuring electrode assembly used in the prior art. In FIG. 1, the measuring electrode assembly has a structure in which a potential difference between an internal electrode 4 immersed in a buffer solution 3 which is enclosed in the glass electrode 2 of an insulated glass tube having the bottom of a pH responsive glass membrane and a external reference electrode 5 of silver and silver chloride is taken out by a pair of lead wires 6, 7 respectively.
In this method, the impedance of pH responsive glass membrane 2 is about 10.sup.8 ohm even in the case of a general electrode; it reaches from 10.sup.9 ohm to 10.sup.10 ohm in the case of the transcutaneous measurement electrode when a fine particle glass is used.
In view of the foregoing, it is necessary to use a preamplifier having an extremely high impedance. But when doing so, if a long lead wire connecting the glass electrode and the preamplifier is used, interfering noise is transmitted by the electrostatic and magnetic induction of the lead wire.
In FIG. 1, 9 denotes a membrane holder bonding a stretched CO.sub.2 permeable polymer membrane 8. 10 denotes a skin heating membrane which provides a body contact surface and an aperture through which measuring gas is carried to said glass electrode and further provides a heater for heating the measurement portion and heat sensitive element controlling said heater to keep the measurement portion at a designated temperature and which supports the membrane holder 9 in a space between said reference electrode 5 and said skin heating member so as to exert pressure on the CO.sub.2 permeable membrane 8.
FIG. 2 shows a cross section of another improved prior art type of transcutaneous CO.sub.2 measurement assembly which is designed to overcome the above defects. A preamplifier 27 is placed at an upper position of the glass electrode. The preamplifier 27 is connected to the internal electrode 24 immersed in a buffer solution 23 which is enclosed in the glass electrode. The glass electrode is made of insulating tubular member 22 bonded by pH responsive glass membrane 21 so as to form its bottom.
The output terminal of the preamplifier 27 is connected to an external measurement main body which is placed apart from the electrode assembly by lead wires 28 with a low output impedance which is produced by the impedance conversion function of the preamplifier. The membrane holder 9 with a CO.sub.2 permeable polymer membrane 8 and a skin heating member having heater 30 and heat sensitive element 31 are the same as those shown in FIG. 1. The skin heating member 10 exerts pressure on the CO.sub.2 permeable membrane interposing electrolyte solution on the outer surface of said glass membrane 21 in the same manner as in FIG. 1. However, the improved prior art assembly as shown in FIG. 2 still has defects, more specifically in the following respects.
(1) The assembly becomes larger in size because it houses a preamplifier at an upper place of the glass electrode, and therefore it is not convenient to use the assembly for prematured and neonatal babies with small body surface.
(2) Because the lead wire from the glass electrode to the preamplifier has a high impedance, this easily takes up noises.
(3) It is required to shield the portion of the lead wire from the glass electrode to the preamplifier.
(4) When the sensor falls or is inverted by the moving of the patient, the buffer solution of the glass electrode runs out, thereby happens disconnection in the lead wire of the glass electrode.
(5) The buffer solution may happen to freeze causing the glass electrode enclosing the buffer solution to break under preservation and/or transportation in very cold climates.
(6) When the assembly is heated for transcutaneous measurement, the buffer solution may become vaporized and form dews because of the uneven temperature distribution over the glass electrode. The composition of the solution changes locally, and abnormal measurement values may appear.
(7) The buffer solution which has leaked out of the glass electrode may deteriorate insulation.
(8) The pH responsive glass membrane is easily broken.
(a) The skin heating member 10 is designed to heat a skin measurement portion of the patient and the glass electrode to maintain both at a designated temperature. Therefore, it is not sufficient to maintain the preamplifier provided outside of the glass electrode at a constant temperature. The changes in temperature of the preamplifier causes drifts in output voltage and deteriorates the measurement precision of the electrode assembly.